1. Field of the Invention
This invention relates to a method of polishing a silicon wafer in which a polishing solution substantially containing no abrasive grain is applied onto a surface of a polishing pad having a given fixed grain bonded abrasive and then the polishing pad is relatively slid to a silicon wafer to polish the surface of the silicon wafer.
2. Description of the Related Art
As the polishing solution used for polishing the surface of the silicon wafer is widely used an alkali solution containing silica particles as loose grain since early times. Also, a wet-type chemical mechanical polishing (mechanochemical polishing) with a polishing solution containing fine silica particles as abrasive grains and a soft synthetic leather polisher is general in the polishing of the silicon wafer. This polishing method is a method of combining the mechanical polishing action of the fine silica particles with the chemical polishing action of the alkaline solution, and is known to provide a mirror surface having excellent smoothness and crystallinity.
However, when the silicon wafer is polished with the polishing solution containing abrasive grains as mentioned above, it is possible to obtain a high processing speed and a certain wafer flatness, but there is a problem that the surface of the silicon wafer is damaged by the abrasive grains during the polishing to cause working strain on the wafer surface.
On the other hand, as a polishing method for preventing the working strain is known a chemical polishing method using an alkali solution substantially containing no abrasive grain as a polishing solution. However, when only the simple chemical polishing is carried out, there is a problem that the shape accuracy of the polished surface is poor, and when this chemical polishing is used in the polishing of the silicon wafer, there is a problem that the polishing speed is dramatically deteriorated since an oxide film naturally growing on the wafer surface cannot be completely removed or the removing time is too long.
In order to solve the above problems, therefore, the polishing process of the silicon wafer is divided into two stages of removing an oxide layer and polishing a silicon surface as disclosed in, for example, JP-A-H09-306881; that is, there is mentioned a so-called two-stage polishing method wherein the removal of the oxide layer is carried out with a polishing solution containing loose abrasive grains and the polishing of the silicon surface is carried out with a polishing solution substantially containing no abrasive grain. By using this method, it is possible to develop effects that the polishing time can be shortened in the removal of the oxide layer and that the occurrence of working strain can be suppressed on the surface of the silicon wafer.
When using the polishing method disclosed in JP-A-H09-306881, however, there is a fear that since it is difficult to control supply or homogenization of the abrasive grains and the polishing solution, the polishing amount becomes relatively large in a part of a silicon wafer while the polishing amount becomes relatively less in the other part thereof and hence the sufficient flatness is not obtained in the silicon wafer after the polishing. This tendency is particularly remarkable in case of a wafer having a large diameter.